Re: [PATCH] iommu/intel: Fix __domain_mapping()'s usage of switch_to_super_page()

[David Woodhouse]

On Tue, 2025-08-26 at 14:38 +0000, Eugene Koira wrote:
> switch_to_super_page() assumes the memory range it's working on is aligned
> to the target large page level. Unfortunately, __domain_mapping() doesn't
> take this into account when using it, and will pass unaligned ranges
> ultimately freeing a PTE range larger than expected.
> 
> Take for example a mapping with the following iov_pfn range [0x3fe400,
> 0x4c0600], which should be backed by the following mappings:

The range is [0x3fe400, 0x4c0600) ? 

>    iov_pfn [0x3fe400, 0x3fffff] covered by 2MiB pages
>    iov_pfn [0x400000, 0x4bffff] covered by 1GiB pages
>    iov_pfn [0x4c0000, 0x4c05ff] covered by 2MiB pages
> 
> Under this circumstance, __domain_mapping() will pass [0x400000, 0x4c05ff]
> to switch_to_super_page() at a 1 GiB granularity, which will in turn
> free PTEs all the way to iov_pfn 0x4fffff.
> 
> Mitigate this by rounding down the iov_pfn range passed to
> switch_to_super_page() in __domain_mapping()
> to the target large page level.
> 
> Additionally add range alignment checks to switch_to_super_page.
> 
> Fixes: 9906b9352a35 ("iommu/vt-d: Avoid duplicate removing in __domain_mapping()")
> Signed-off-by: Eugene Koira <eugkoira@xxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> ---
>  drivers/iommu/intel/iommu.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/iommu/intel/iommu.c b/drivers/iommu/intel/iommu.c
> index 9c3ab9d9f69a..dff2d895b8ab 100644
> --- a/drivers/iommu/intel/iommu.c
> +++ b/drivers/iommu/intel/iommu.c
> @@ -1575,6 +1575,10 @@ static void switch_to_super_page(struct dmar_domain *domain,
>  	unsigned long lvl_pages = lvl_to_nr_pages(level);
>  	struct dma_pte *pte = NULL;
>  
> +	if (WARN_ON(!IS_ALIGNED(start_pfn, lvl_pages) ||
> +		    !IS_ALIGNED(end_pfn + 1, lvl_pages)))
> +		return;
> +
>  	while (start_pfn <= end_pfn) {
>  		if (!pte)
>  			pte = pfn_to_dma_pte(domain, start_pfn, &level,
> @@ -1650,7 +1654,8 @@ __domain_mapping(struct dmar_domain *domain, unsigned long iov_pfn,
>  				unsigned long pages_to_remove;
>  
>  				pteval |= DMA_PTE_LARGE_PAGE;
> -				pages_to_remove = min_t(unsigned long, nr_pages,
> +				pages_to_remove = min_t(unsigned long,
> +							round_down(nr_pages, lvl_pages),
>  							nr_pte_to_next_page(pte) * lvl_pages);
>  				end_pfn = iov_pfn + pages_to_remove - 1;
>  				switch_to_super_page(domain, iov_pfn, end_pfn, largepage_lvl);

I'm mildly entertained by the fact that the *only* comment in this
block of code is a lie. Would you care to address that while you're
here? Maybe the comment could look something like...

			/* If the new mapping is eligible for a large page, then
			 * remove all smaller pages that the existing pte at this
			 * level references.
			 * XX: do we even need to bother calling switch_to_super_page()
			 * if this PTE wasn't *present* before?
			 */

I bet it would benefit from one or two other one-line comments to make
it clearer what's going on, too...

But in general, I think this looks sane even though this code makes my
brain hurt. Could do with a test case, in an ideal world. Maybe we can
work on that as part of the generic pagetable support which is coming?

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